The field of this invention relates to cyclonic separation of solids from liquids or liquids from liquids.
Cyclones have been in use in separation applications in a variety of industries for many years. Typically, these devices have a cylindrical body tapering to an underflow outlet, with a tangential or involute entrance and a centrally located end connection for the overflow fluids at the head end of the hydrocyclone. These devices are used to separate fluids of different densities and/or to remove solids from an incoming stream of a slurry of liquid and solids, generally concentrating the solids in the underflow stream.
Over the years, many efforts have been undertaken to optimize the performance of hydrocyclones. Performance increase could be measured as an increase in throughput without material sacrifice in the degree of separation desired for a given operating pressure drop. An alternate way to measure improved performance is to increase the separation efficiency for a given inlet flow rate and composition.
In the past, a cyclone has been provided with a single ramp presenting a generally planar face extending at a relatively shallow angle to a radial plane of the hydrocyclone and thus inclined toward the underflow end of the hydrocyclone. Thus, when the fluid enters from the inlet, the fluid swirls about the axis of the chamber, with the back wall imparting to the mixture an axial velocity component in the direction toward the underflow outlet. This design is illustrated in PCT application WO97/05956. Also relevant to a general understanding of the principles of operation of hydrocyclones are PCT applications WO97/28903, WO89/08503, WO91/16117, and WO83/03369; U.K. specification 955308; U.K. application GB 2230210A; European applications 0068809 and 0259104; and U.S. Pat. Nos. 2,341,087 and 4,778,494.
In the past, a single helix of a uniform pitch was used to present an inclined surface to the incoming mixture. The inclined surface terminated at a step after the incoming mixture has undergone a complete revolution within the separating chamber. Thus, this prior design, illustrated in PCT application WO97/05956, took the entire incoming fluid stream and imparted a generally uniform velocity axial component to the generally helical flowpath of that entire incoming stream.
However, applicants"" detailed studies of the axial flow of the fluid after it enters the hydrocyclone have revealed that, as viewed in a radial direction from the longitudinal centerline of the hydrocyclone, a preferred flow pattern would be nonuniform, with the greatest velocity being adjacent the peripheral wall of the hydrocyclone. Moving in radially from the outer periphery toward the longitudinal axis, the axial velocity component of the fluid mass decreases until it undergoes a reversal in direction representing the fluid stream that is heading toward the overflow outlet.
Accordingly, in seeking further capacity or efficiency improvements, one of the objectives of the present invention was to minimize turbulence internal to the hydrocyclone and thereby increase its performance. The capacity improvement was achieved by recognizing that in order to minimize turbulence, the incoming fluid stream should be driven axially at different velocities, depending on the radial placement of the stream within the body. Accordingly, the objective of improving throughput and/or separation efficiency has been accomplished in the present invention by recognizing this need to reduce turbulence and accommodating this performance-enhancing need by a specially designed back wall ramp featuring multiple side-by-side spiraling slopes, the steepest slope being furthest from the longitudinal axis with adjacent slopes becoming shallower as measured radially inwardly toward the longitudinal axis. Those skilled in the art will more fully appreciate the significance of the present invention by a review of the detailed description of a preferred embodiment thereof below.
An improvement is made in the efficiency and/or throughput of a hydrocyclone by providing a back wall which imparts a greater axial velocity component to the fluids at the periphery as measured radially from the longitudinal axis of the hydrocyclone and a lesser axial velocity component to portions of the incoming fluid stream closer to the longitudinal axis of the hydrocyclone. More particularly, the back wall should correspond generally to the swirl pattern within the hydrocyclone, a combination of axial and tangential velocity components, to enable the incoming fluid stream to reach the desired flow pattern more quickly and efficiently than otherwise possible.